Solvents for the selective removal of H2 S from gases containing both H2 S and CO2

ABSTRACT

A novel class of solvents is described that have improved selectivity for H 2  S as compared to CO 2 . The solvents are based on adding suitable second partially miscible solvent to known solvents. In specific, it is shown that adding 20% of dodecane to NMP (normal methyl pyrolidinone) increases the selectivity of the solvent for H 2  S as compared to CO 2  by 50%. This leads to important process improvements and reduction in the cost of the removal process.

BACKGROUND OF THE INVENTION

Field of the Invention

Physical solvents such as methanol, NMP (normal methyl pyrolidinone) [U.S. Pat. No. 3,103,411 and U.S. Pat. No. 4,208,382]selexol, propylene carbonate [U.S. Pat. No. 2,926,751]and others are widely used for the removal of H₂ S from gases such as natural gas and syngas from coal gasification. In such cases the solvent is contacted counter currently with the gas and the solvent is then heated and later stripped to remove one of the dissolved gases, H₂ S [U.S. Pat. No. 4,198,388]. In many of those applications, CO₂ is also present and it is important to minimize the CO₂ that is released together with the H₂ S in the stripping of the solvent. This is due to the fact that high concentration of CO₂ would make complete conversion of the H₂ S in a chemical process more difficult. This complexes the design of the extraction column, and there are a number of commercial designs that have solved that problem [Kohl and Riesenfeld, 1985, Newman, 1985]. The details of these designs are not relevant to our inventions. It is enough to know that satisfactory design exists.

One important and well known feature of these designs is that they require a solvent which is highly selective for H₂ S as compared to CO₂. This is especially important if the concentration of CO₂ is much higher than that of H₂ S.

It is obvious to those skilled in the art that any improvement in selectivity has great value in making the cost of separating and extending the range of relative conditions over which the method is commercially viable. The field of the present invention is the description of a method which simplifies the search for improved solvents as well as specific examples of solvent mixtures with greatly enhanced selectivity properties.

Partially Miscible Solvent Mixtures With A Critical Point of Miscibility.

Let us consider a binary mixture of solvents which have a critical point of miscibility. The meaning of such a critical point can be explored by a phase diagram (see FIG. 1).

This figure describes the concentration of component A and B as a function of temperature. The region marked "one phase" represents conditions of complete miscibility of components A and B, whereas regions marked "two phase" correspond to separation into two phases. When the phase diagram shows a maximum (FIG. 1a), it is called an upper critical solution temperature (UCST), whereas a minimum (FIG. 1b) corresponds to a lower critical solution temperature (LCST).

Liquids posses also a vapor-liquid critical point. Above the critical temperature, there is only one fluid phase independent of pressure. In the last twenty years, it was found that near this critical point there are some very interesting thermodynamic properties, [Paulaitis, et al., 1983]. Those unique properties have resulted in the development of separation processes named supercritical extraction. However, that field should not be confused with the present art.

SUMMARY OF THE INVENTION

The present invention relates to a process for selectively removing H₂ S from a gas stream containing both H₂ S and CO₂ which comprises:

contacting said gas stream with a solvent comprising a mixture of N-methylpyrrolidone (NMP) and dodecane, said mixture having a critical point of miscibility, to selectively dissolve H₂ S in said solvent thereby providing a gas stream of reduced H₂ S content and a solvent containing dissolved H₂ S.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a phase diagram of an immiscible binary system with an upper critical solution temperature.

FIG. 1(b) is a phase diagram of an immiscible binary system with a lower critical solution temperature.

FIG. 2 is a phase diagram of an acetonitrile/water system.

FIG. 3 is a phase diagram of an NMP/dodecane system.

FIG. 4 is a phase diagram of an NMP/methyl-cyclohexane system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Description of the Solvents Proposed

Ludmer et al. [1987]have previously shown that a class of solvent mixtures in the vicinity of their critical point of miscibility obtain an increased solubility for some solids. Contrary however to our previous publication, we have found here that the maximum effect is not in the neighborhood of the critical point. In fact it is often more pronounced in a region where two liquid phases exist. It was detected in some systems to have a maximum effect close to the phase transition and not necessarily at the critical composition.

Let us now look as to what available evidence and theory predict for the solubility of a gas in a solvent mixture as a function of composition. If one uses, for example, a mixture of two solvents, then solubility of a gas at low gas pressure can be expressed as follows. If H is the Henry constant defined by

    H.sub.1 -p.sub.i /S.sub.i                                  (1)

where p_(i) is the partial pressure of the gas compound in equilibrium with the solution and x_(i) the molar fraction of the dissolved gas in the mixed solution, then the solubility of the gas in an ideal mixture and low gas concentration can be computed [O'Connell, 1971]by Krischevsky [1937]equation

    lnH.sub.3,m -S.sub.1 lnH.sub.3,1 +X.sub.2 lnH.sub.3,2      (2)

Where 3 denote for the dissolved gas, 1 and 2 for the solvents and m for the mixture. H₃,1 and H₃,2 are the Henry constants in the pure solvents. Equation 2 is derived by computing the thermodynamic potential in an ideal mixture of two solvents with no interactions. Obviously, pariially miscible solvents have strong interactions, which can be indicated by either positive (higher Henry Constant) or negative (decreased Henry's Constant) deviations compared to Eq. 2. However, the solubility of different gases in the same CST forming mixture can result in different deviation in magnitude, and in some solvent gas systems, also in the deviation sign. These phenomena can result in increased selectivity when compared to the pure constituents of the mixture.

In tables 1 to 4, we present some typical gas solubility results for selected mixtures of partially miscible solvents both above and below the critical point of miscibility, and as a reference, table 5 summarizes the solubility of the gases in the pure solvents. The example solvents being three solvent pairs NMP-dodecane, NMP-methyl-cyclohexane and acetonitrile-water. In FIGS. 2 to 4 we show the diagrams of the above solvent pairs.

We found strong effects, as far as selectivity is concerned, in all of the above mentioned solvent mixtures. However, only in the NMP-dodecane and the acetonitrile-water mixtures is the effect useful for our purposes. In the NMP-methyl-cyclohexane case the nonlinear effect favors CO₂ over H₂ S. At present, we have no method that would predict the direction of the effect in a specific solvent mixture All what we have established is that the effects are surprisingly large and that such effects have never been reported in the literature. As one can notice from tables 3 and 4, the maximum selectivity is not close to the CST but rather in the two phase region. The total solubility in these cases is reduced but still acceptable. The increase in selectivity is technically and economically of some importance.

We do not claim that all mixtures of partially miscible solvents with a critical point of solubility will have these effects or that adding a second solvent that forms a critical mixture will always increase selectivity. What we claim is that addition of such a second solvent can in some systems lead to drastic improvements in selectivity as illustrated by the above examples. However, it should be pointed out that some solvent systems may favor CO₂ over H₂ S. The references cited above are cited in greater detail below.

Description of the Proposed Process

As said before, removal of acid compounds such as H₂ S and CO₂ from gas streams by physical solvents is common commercial practice, and proven process equipments and flow-sheets are available.

In our process modification, the type of equipment used remains the same. What our proposed process modification does, is to increase the selectivity of the gas removal by improving the selectivity of the solvent. As an example we provide data for removing H₂ S in the presence of CO₂. That could in principal be achieved in two ways. One is to find a better, more selective solvent. A solvent mixture of the type described here could serve this purpose and we will give an example later. A second way is to take the primary solvent that is used in the process and add to it a second solvent that forms with it a solution with a critical point of miscibility. An example of such a type is NMP (normal methyl pyrolidinone) which is used in the Purisol process. If 8% (17% by volume) dodecane is added to it then at a temperature of about 35° C., the selectivity is increased by 60%.

The solubility and selectivity for this example are given in tables 1 to 4. We note that the mixture with 8% dodecane and at 35° C. is not at the critical composition or temperature which are 50% dodecane, 50% NMP at temperature of 78° C., but in the NMP rich phase. However, similar effects could also be observed in the one phase region. We argue that it is the nature of such mixtures to promote non-linear effects over a wide range of concentrations and our claim is over the whole region where such effects persist.

A second example is the case where acetonitrile is the primary solvent. It is not commonly used in gas purification, but has excellent properties. Here addition of 30% (by volume) water at 5° C. leads to a greatly improved solvent with higher selectivity for H₂ S compared to CO₂, as can be seen from the data in tables 3 and 4.

Solvent mixtures with a critical point are also formed if more than one additional solvent is added. Even solid compounds can be added to change the properties of the solvent mixture as well as

the composition and temperature of the critical point. Such cases do not substantially modify the basic concept of our invention and are therefore part of the improved process method claimed.

There is a large class of such solvent mixtures both binary and ternary that are potential candidates for improved gas absorption and the above solvent systems are considered as examples of a practical realization of our model concept.

    ______________________________________                                         Reference Cited                                                                ______________________________________                                         UNITED SATE PATENTS                                                            2,926,751    Kohl et al.   Mar. 1, 1960                                        3,103,411    Werner        Sep. 10, 1963                                       4,198,388    Laslo ey al.  Apr. 15, 1980                                       4,208,382    Blume et al.  Jun. 17, 1980                                       OTHER PUBLICATIONS                                                             Kohl, A. and Riesenfeld, F., "Gas Purification",                               Fourth ed., Gulf Publication Company, 1985.                                    Krischevsky, I., R., Zh. Fiz. Khim., Vol. 9, 41, 1937.                         Ludmer, Z., Shinnar, R. and Yakhot, V., "Solubility                            in Binary Mixtures at the Immiscibility Critical Point",                       AIChE J., Vol. 33, 1776-1780, 1987.                                            Newman A. S. (editor), "Acid and Sour Gas Treating                             Processes", Gulf Publication Company, 1985.                                    O'Connell, J. P., "Molecular Thermodynamics of                                 Gases in Mixed Solvents", AIChE J., Vol. 17,                                   658-663, 1971.                                                                 Paulaitis, M. E., et al., "Supercritical Fluid Extraction",                    Rev. Chem. Eng., Vol. 1, 179,                                                  ______________________________________                                    

                                      TABLE 1                                      __________________________________________________________________________     Solubility of carbon dioxide and hydrogen sulfide (at 1 atm                    partial pressure) in the one phase region.                                                            Solubility Ratio                                                               Mixture/                                                                             Mixture/                                          Solvents                                                                               Solute                                                                              Temp.     Good  Poor  Mixture/                                    System  (gas)                                                                               °C.                                                                          Solub..sup.1                                                                        Solvent                                                                              Solvent                                                                              Raoult's.sup.2                              __________________________________________________________________________     Acetonitrile 10   4.40 0.464 3.451 1.973                                       (good solv.) +                                                                              15   3.87 0.444 3.686 2.026                                       Water   CO.sub.2                                                                            20   3.54 0.426 3.933 2.082                                       (poor solv.) 25   3.21 0.401 4.280 2.169                                       (0.609).sup.3                                                                               15   16.30                                                                               0.534 5.552 2.876                                                    20   13.85                                                                               0.519 5.368 2.778                                               H.sub.2 S                                                                           45   7.81 0.462 5.049 2.543                                                    60   5.23 0.400 4.144 2.117                                       NMP     CO.sub.2                                                                            78   0.84 0.467 1.024 0.832                                       (good solv.)                                                                   Dodecane                                                                               H.sub.2 S                                                                           78   11.58                                                                               0.739 5.514 2.305                                       (poor solv.)                                                                   (0.297).sup.3                                                                  NMP          18   4.40 0.894 2.018 1.401                                       (good solv.) 20   4.20 0.878 1.974 1.372                                                    30   3.50 0.854 1.938 1.338                                       Methyl- CO.sub.2                                                                            40   2.92 0.859 1.879 1.352                                       Cyclohexane  50   2.39 0.885 1.833 1.317                                       (0.412).sup.3                                                                               60   1.90 0.805 1.753 1.228                                                    70   1.42 0.703 1.632 1.107                                               H.sub.2 S                                                                           10   53.55                                                                               0.729 3.688 2.031                                                    20   45.08                                                                               0.717 3.840 2.009                                                    30   33.52                                                                               0.638 3.520 1.774                                                    40   24.58                                                                               0.598 3.208 1.598                                                    50   18.52                                                                               0.593 3.046 1.515                                                    60   13.99                                                                               0.593 3.937 1.461                                                    70   11.00                                                                               0.564 3.013 1.430                                       __________________________________________________________________________      .sup.1 Volume of the gas at standard condition over solutefree solvent         volume.                                                                        .sup.2 Ideal mixture (eq. 2).                                                  .sup.3 Volume fraction of the good solvent                               

                                      TABLE 2                                      __________________________________________________________________________     Solubility of carbon dioxide and hydrogen sulfide in critical mixtures         (at 1 atm partial pressure) in the two phase region.                                                  Mixture/                                                                            Mixture/                                           Solvents      Temp.    Good Poor  Mixture/                                     System  Phase °C.                                                                         Solub..sup.1                                                                        Solvent                                                                             Solvent                                                                              Raoult's.sup.2                               __________________________________________________________________________     Aetonitrile                                                                            ACN   -5  11.84                                                                               1.024                                                                               6.072 2.349                                        (good solv.) +                                                                         rich   0  8.76 0.826                                                                               5.078 2.246                                        Water          3  7.25 0.706                                                                               4.560 2.287                                        (poor solv.)   5  5.48 0.545                                                                               3.651 2.148                                        +CO.sub.2 (gas)                                                                        Water -5  3.21 0.277                                                                               1.644 1.367                                                rich   0  3.44 0.324                                                                               1.992 1.555                                                       3  3.92 0.382                                                                               2.465 1.790                                                       5  5.33 0.530                                                                               3.553 2.141                                                Average                                                                              -5  8.00 0.692                                                                               4.103 2.548                                                       0  6.40 0.604                                                                               3.710 2.974                                                       3  5.78 0.563                                                                               3.635 2.189                                                       5  5.41 0.538                                                                               3.607 2.147                                        +H.sub.2 S (gas)                                                                       ACN rich                                                                              5  28.90                                                                               0.858                                                                               7.298 2.555                                                Water rich                                                                            5  8.95 0.266                                                                               2.260 1.731                                                Average                                                                               5  21.44                                                                               0.636                                                                               5.414 3.017                                        NMP     NMP   20  3.12 0.652                                                                               2.940 0.735                                        (good solv.) +                                                                         rich  35  2.09 0.572                                                                               2.173 0.635                                        Dodecane      45  1.53 0.553                                                                               1.776 0.628                                        (poor solv.)  65  1.18 0.538                                                                               1.426 0.658                                        +CO.sub.2 (gas)                                                                        Dodecane                                                                             20  1.06 0.222                                                                               1.000 0.855                                                rich  35  0.97 0.265                                                                               1.007 0.876                                                      45  0.89 0.321                                                                               1.031 0.909                                                      65  0.83 0.379                                                                               1.005 0.863                                                Average                                                                              20  1.49 0.312                                                                               1.406 0.738                                                      35  1.20 0.329                                                                               1.250 0.710                                                      45  1.02 0.369                                                                               1.186 0.732                                                      65  0.91 0.414                                                                               1.096 0.743                                        +H.sub.2 S (gas)                                                                       NMP   20  55.60                                                                               0.885                                                                               8.633 1.226                                                rich  35  44.50                                                                               0.931                                                                               8.768 1.340                                                      50  32.24                                                                               0.856                                                                               7.694 1.187                                                      65  19.94                                                                               0.924                                                                               6.453 1.388                                                Dodecane                                                                             20  9.02 0.144                                                                               1.400 1.153                                                rich  35  9.49 0.198                                                                               1.870 1.490                                                      50  10.03                                                                               0.266                                                                               2.394 1.896                                                      65  11.27                                                                               0.522                                                                               3.647 2.621                                                Average                                                                              20  22.94                                                                               0.365                                                                               3.562 1.451                                                      35  20.20                                                                               0.422                                                                               3.981 1.597                                                      50  17.96                                                                               0.422                                                                               3.981 1.721                                                      65  14.08                                                                               0.653                                                                               4.558 2.003                                        NMP     NMP   10  5.56 1.010                                                                               2.269 1.304                                        (good solv.) +                                                                         rich  15  5.03 0.978                                                                               2.206 1.315                                        Methyl- MCH   10  3.67 0.667                                                                               1.498 1.380                                        cyclohexane                                                                            rich  15  3.50 0.680                                                                               1.534 1.300                                        (poor solv.)                                                                           Average                                                                              10  5.10 0.927                                                                               2.080 1.451                                        +CO.sub.2 (gas)                                                                              15  4.66 0.906                                                                               2.042 1.419                                        __________________________________________________________________________      .sup.1 Volume of the gas at standard condition over solutefree solvent         volume.                                                                        .sup.2 Ideal mixture (eq. 2)                                             

                                      TABLE 3                                      __________________________________________________________________________     Selectivity of hydrogen sulfide in respect to carbon dioxide in critical       mixtures (at 1 atm partial pressure) in the one phase region.                                              Selectivity Ratio                                                  Volume      Mixture/                                                                             Mixture                                      Solvents    Temp.                                                                              Fraction    Good  Poor                                         System      °C.                                                                         (good solv.)                                                                           Selc..sup.1                                                                        Solvent                                                                              Solvent                                      __________________________________________________________________________     Acetonitrile                                                                               15  0.609   4.212                                                                              1.203 1.506                                        (good solvent) +                                                                           20          3.912                                                                              1.219 1.365                                        Water (poor solvent)                                                           NMP (good solvent) +                                                                       78  0.297   13.786                                                                             1.582 5.383                                        Dodecane                                                                       (poor solvent)                                                                 NMP (good solvent) +                                                                       20  0.412   10.733                                                                             0.817 1.946                                        Methyl-cyclohexane                                                                         30          9.577                                                                              0.748 1.816                                        (poor solvent)                                                                             50          7.749                                                                              0.670 1.662                                                    60          7.363                                                                              0.736 1.676                                                    70          7.746                                                                              0.802 1.846                                        __________________________________________________________________________      .sup.1 Ratio- mole H.sub.2 S to mole CO.sub.2 absorbed                   

                                      TABLE 4                                      __________________________________________________________________________     Selectivity of hydrogen sulfide in respect to carbon dioxide in critical       mixtures (at 1 atm partial pressure) in the two phase region.                                              Selectivity Ratio                                                  Volume      Mixture/                                                                             Mixture/                                     Solvents    Temp.                                                                              Fraction    Good  Poor                                         Systems Phase                                                                              °C.                                                                         (good solv.)                                                                           Selec..sup.1                                                                       Solvent                                                                              Solvent                                      __________________________________________________________________________     Acetonitrile                                                                           ACN  5  0.70    5.277                                                                              1.578 2.00                                         (good solvent) +                                                                       rich                                                                   Water   Water                                                                               5  0.45    1.679                                                                              0.502 0.636                                        (poor solvent)                                                                         rich                                                                   NMP     NMP 20  0.85    17.84                                                                              1.358 2.937                                        (good solv.) +                                                                         rich                                                                               35  0.83    21.33                                                                              1.628 4.040                                        Dodecane                                                                               Dodec.                                                                             20  0.06    8.51                                                                               0.648 1.401                                        (poor solv.)                                                                           rich                                                                               35  0.07    9.81                                                                               0.748 1.858                                        __________________________________________________________________________      .sup.1 Ratio  mole H.sub.2 S to mole CO.sub.2 absorbed.                  

                  TABLE 5                                                          ______________________________________                                         Solubility of carbon dioxide and hydrogen sulfide                              (at 1 atm and 20° C.) in pure solvents.                                 Solvent         Solute (gas)                                                                              Solubility.sup.1                                    ______________________________________                                         Acetonitrile    CO.sub.2   8.31                                                                H.sub.2 S  26.69                                               NMP             CO.sup.2   4.78                                                                H.sub.2 S  62.82                                               Methyl-cyclohexane                                                                             CO.sub.2   2.13                                                                H.sub.2 S  11.74                                               Dodecane        CO.sub.2   1.06                                                                H.sub.2 S  6.44                                                Water           CO.sub.2   0.89                                                                H.sub.2 S  2.58                                                ______________________________________                                          .sup.1 Volume of the gas at standard condition over solutefree solvent         volume.                                                                   

What is claimed is:
 1. A process for selectively removing H₂ S from a gas stream containing both H₂ S and CO₂ which comprises:contacting said gas stream with a solvent comprising a mixture of N-methylpyrrolidone (NMP) and dodecane, said mixture having a critical point of miscibility, to selectively dissolve H₂ S in said solvent thereby providing a gas stream of reduced H₂ S content and a solvent containing dissolved H₂ S.
 2. The process of claim 1 which further comprises:removing said dissolved H₂ S from said solvent containing dissolved H₂ S.
 3. The process of claim 1 wherein said mixture comprises about 20% of dodecane. 